Reuters earlier quoted the Turkish ruling party as saying the investigation would be comprehensive and that the government's sensitivity about the case was at the "highest level". The AK Party said Mr Khashoggi's whereabouts would be uncovered.

On Wednesday, the Turkish foreign ministry summoned Saudi Arabia's ambassador and asked for an explanation about the disappearance.

What have the Saudis said?

Prince bin Salman told Bloomberg: "He's a Saudi citizen and we are very keen to know what happened to him. And we will continue our dialogue with the Turkish government to see what happened to Jamal there.

Image copyrightAFPImage caption
The Saudi consulate said Khashoggi left after completing paperwork

"My understanding is he entered and he got out after a few minutes or one hour. I'm not sure. We are investigating this through the foreign ministry to see exactly what happened at that time.

"The premises are sovereign territory, but we will allow them to enter and search and do whatever they want to do. If they ask for that, of course, we will allow them. We have nothing to hide."

When asked if Mr Khashoggi faced charges in Saudi Arabia, the crown prince said his country would need to know where he was first.

What happened on Tuesday?

Mr Khashoggi went to the consulate to obtain a document certifying he had divorced his ex-wife, so that he could marry his Turkish fiancée, Hatice, who went with him to the building and waited outside, but did not see him leave.

She said that he was "stressed and sad" that he was forced to go to the building.

He was required to surrender his mobile phone, which is standard practice in some diplomatic missions.

Hatice said he left the phone with her and told her to call an adviser to Turkish President Recep Tayyip Erdogan if he did not return.

She said she waited for him outside the consulate from about 13:00 (10:00 GMT) until after midnight and did not see him leave. She returned when the consulate reopened on Wednesday morning.

Why might Saudi Arabia want to hold Khashoggi?

The 59-year-old journalist is one of the most prominent critics of the crown prince, who has unveiled reforms praised by the West while carrying out an apparent crackdown on dissent, which has seen human and women's rights activists, intellectuals and clerics arrested, and waging a war in Yemen that has triggered a humanitarian crisis.

A former editor of the al-Watan newspaper and a short-lived Saudi TV news channel, Mr Khashoggi was for years seen as close to the Saudi royal family. He served as an adviser to senior Saudi officials.

After several of his friends were arrested, his column was cancelled by the al-Hayat newspaper and he was allegedly warned to stop tweeting, Mr Khashoggi left Saudi Arabia for the US, from where he wrote opinion pieces for the Washington Post and continued to appear on Arab and Western TV channels.

"I have left my home, my family and my job, and I am raising my voice," he wrote in September 2017. "To do otherwise would betray those who languish in prison. I can speak when so many cannot."